Medical X-ray imaging has a long history. The earliest techniques were based on transilluminating the object being imaged. In transillumination, all the anatomies of the volume being imaged possibly overlapping in the direction of radiation are imaged on film on top of each other. Concerning layer imaging, i.e. a so-called tomographic imaging, on the other hand, one may get in the image being formed a desired layer of the object to become imaged more clearly by causing blurring of the other layers of the object. Depending on the imaging procedure, blurring is accomplished by changing the relative position of the imaging means and the object in a controlled manner during the imaging event either during irradiation or between individual irradiations. Especially along with advancement of computers and digital imaging, a great number of different tomographic imaging techniques and devices have been developed.
In the field of odontology, in addition to intra-oral and cephalometric imaging which are simpler as far as imaging technology is concerned as they are realized by transillumination imaging one generally uses, among other things, a so-called panoramic imaging in which, typically, a layer comprising the whole dental arch is imaged on a plane. In conventional film-based panoramic imaging, one scans over the dental arch with a narrow beam such that the centre of rotation of a turnable arm part, substantially to the opposite ends of which the imaging means have been positioned, is transferred linearly while the arm part is turned and the film moving together with the arm part is transferred through the narrow beam produced by a radiation source with a speed fulfilling the imaging condition of the imaging procedure in question. In digital panoramic imaging, the frequency at which image data is read from the sensor during an imaging scan corresponds this transfer speed of the film.
One has also begun to apply computer (or computed) tomography (CT), used earlier predominantly in hospital environment, in the field of odontology. As such, one is not able to transfer these massive and expensive CT apparatuses used in hospitals to a typical dental clinic environment, already on account of the size of the apparatuses but especially on account of their price.
Imaging-technically, several different CT technologies are known today. In CT imaging, the volume to be imaged is irradiated from different directions and, from the data thus acquired, a desired two- or three-dimensional image is reconstructed afterwards. In principle, using this kind of technology one is also able to reconstruct, among other things, a two-dimensional image outspread on a plane of a part of the dental arch or, if desired, of the whole dental arch. As far as principles of computed tomography and its different applications are concerned, a reference can be made to the literature on the art, such as to Computed Tomography: Principles, Design, Artifacts and Recent Advantages, Jian Hsich, SPIE PRESS, 2003, Bellingham, Wash., USA.
One form of computed tomography is the so-called cone beam CT (CBCT) in which one uses, as a distinction from the narrow beam used e.g. in panoramic imaging and conventional CT imaging, a beam substantially the size of the dimensions of the volume to be imaged and, respectively, instead of a slot sensor, a detector the size of which corresponds the size of the beam in question. Compared to several more conventional CT imaging technologies, the CBCT technology is able to provide significantly smaller radiation doses and shorter imaging times.
A typical starting point in some of the CT solutions outlined and realized for odontology has been arranging the imaging means to a relatively massive, stable support construction in which the patient is positioned in a sitting position on a chair in between the imaging means, and the possible relative motions of the patient location and the imaging means, for positioning the imaging means ready for imaging a desired volume, are realized by moving the chair. On the other hand, e.g. U.S. Pat. No. 6,118,842 outlines a structure based on a traditional dental panoramic apparatus by which one is able to both turn the imaging means with respect to the centre of rotation and to change the position of the centre of rotation by means of a moving mechanism of the arm part comprising the imaging means. The dimensions of this apparatus and those of the detector used in it enable gathering information for reconstructing a volume of a certain portion of the skull but, in case of desiring to acquire larger, several or e.g. adjacent volumes reconstructed by the apparatus, one has to repeat the imaging by first arranging the relative position of the object and the imaging means according to the new target area to be imaged.
The size of the volume being imageable by one rotation of the imaging means can be increased with the so-called offset imaging. One known manner to realize such imaging is to arrange the imaging sensor movable before imaging to such a position with respect to the target area in which, when rotating the imaging means, at each moment of time only part of the area desired to be imaged is in the beam but, when the whole rotation has been completed, all of the partial areas of the target area have been covered at an angle range of essentially at least 180 degrees. A corresponding result is also reached by moving the position of the centre of rotation of the imaging means, such as in connection with an apparatus described in U.S. Pat. No. 7,486,759, which specification is attached hereto to also more comprehensively depicting the principles of offset imaging according to prior art.
In the above-mentioned U.S. Pat. No. 7,486,759, one essential idea of the solution is to realize an imaging scan of 360 degrees with one exposure of extended duration. This kind of exposure loads the radiation source, which may prove to be problematic especially in such combination x-ray apparatus being originally designed for use that loads the source of radiation less.
An object of the present invention and its preferable embodiments is to provide novel solutions for imaging a greater volume by one imaging than what is possible when the imaging is realized in a conventional manner by using an arm part, in which are arranged at a distance from each other a source of radiation and a receiver of image information, and when both the centre of rotation of the arm part in question and the central axis of the beam are arranged to travel and remain for the whole duration of the imaging process in the middle of the area desired to be imaged.